1. Field of the Invention
This invention relates to articles with self-cleaning coatings. In particular, this invention relates to methods of making photocatalytically-activated self-cleaning (xe2x80x9cPASCxe2x80x9d) coatings on substrates.
2. Discussion of the Background
For both practical and aesthetic reasons, it is often desirable to prevent the buildup of organic and inorganic contamination (i.e., xe2x80x9cdirtxe2x80x9d) on surfaces. Many techniques have been developed over the years to xe2x80x9ccleanxe2x80x9d surfaces by removing accumulations of dirt. Long ago it was discovered that most inorganic dirt can be washed away by applying water to a surface. The addition of soap to water has been found to remove most organic dirt as well.
However, cleaning articles by washing with soap and water is a time consuming process. Even careful washing can leave behind unacceptable amounts of dirt on surfaces of transparent substrates, such as glass, that can interfere with visibility.
Recently, PASC coatings have been developed that, after activation by exposure to ultraviolet (UV) radiation, promote the self-cleaning of coated articles upon rinsing with water. With such coatings, exterior glass surfaces can be cleaned simply by exposure to rain.
The ability of PASC coatings to shed dirt upon exposure to water is currently the subject of intense research. Upon activation by exposure to UV radiation, PASC coatings appear to function both photocatalytically and hydrophilically to remove dirt. Via a photocatalytic effect, the PASC coating causes the bonding of dirt to the coated substrate to break down. Via a hydrophilic effect, the PASC coating increases the ability of water to wet the PASC coating and to slip between dirt and the PASC coating to lift away the dirt.
Activated PASC coatings are characterized as being xe2x80x9csuperhydrophilicxe2x80x9d as a result of the extremely low contact angle between water and the PASC coating. The contact angle of water with inorganic materials, such as glass, is generally 20-40 degrees. The contact angle of water with typical organic materials, such as resins, is 70-90 degrees. The contact angle of water with hydrophobic resins, such as silicone resin and fluorocarbon polymers, is more than 90 degrees. In contrast, the contact angle of UV activated PASC coatings is less than 20 degrees, and often less than 10 degrees, indicative of the propensity of water to spread out over an activated PASC coating. UV irradiation of a PASC coating can cause the water contact angle to approach zero.
The low contact angle of water on an activated PASC coating, in addition to promoting the removal of dirt, reduces the occurrence of water droplets such as fog on the PASC coating. In addition, because water spread out in a film has more surface area than a droplet, water evaporates from a PASC coating faster than from a conventional, non-superhydrophilic, surface.
U.S. Pat. No. 6,027,766 discloses the formation of PASC films of, preferably, titanium oxide by chemical vapor deposition, spray pyrolysis or magnetron sputtering on glass substrates. The ""766 patent discloses that in the chemical vapor deposition and spray pyrolysis processes, the substrate is heated to above about 400xc2x0 C. to cause metal-containing precursors to decompose and form a PASC coating on the substrate. In the sputtering process, the ""766 patent discloses that during or after coating a substrate the substrate is heated to about 400 to about 600xc2x0 C. to form the PASC coating.
U.S. Pat. No. 6,103,363 discloses xe2x80x9cdirt-repellentxe2x80x9d semiconductive metal oxide coatings, such as titanium oxide, on glass substrates. The ""363 patent discloses that the titanium oxide coatings are obtained by decomposition of titanium precursors by liquid pyrolysis, powder pyrolysis, and pyrolysis in the vapor phase (e.g., chemical vapor deposition). The titanium oxide coatings are also obtained by sol-gel techniques and by vacuum techniques such as reactive or non-reactive cathodic sputtering. The ""363 patent recommends subjecting a coating with a photocatalytic property, after deposition, to an annealing heat treatment to improve the degree of crystallization of the coating.
However, post-deposition heat treatments to crystallize PASC coatings are time-consuming and expensive. Furthermore, excessive and/or prolonged heat treatment can adversely affect the PASC coating as well as the quality of the substrate on which the PASC coating is deposited. The photocatalytic and self-cleaning activity of a PASC coating can be significantly reduced or destroyed by the presence of small amounts of alkali metal ions such as sodium. These ions can diffuse during annealing heat treatments into the PASC coating from a substrate such as a soda-lime glass. Sodium ions may inhibit or destroy the photocatalytic activity of a TiO2 PASC coating.
To prevent alkali metal ion diffusion into a PASC coating from a substrate, diffusion barriers have been introduced between the substrate and the PASC coating. Alkali metal ion diffusion barriers are particularly necessary when post-deposition annealing heat treatments are used to form PASC coatings.
There is a need for a method of making a UV activated PASC coating that does not require heating, during or after deposition, to form an effective self-cleaning surface.
The present inventors have discovered that activated PASC coatings with self-cleaning characteristics can be made, surprisingly, without conventional heat treatments during and/or after deposition, if the PASC coating is made by sputtering an ultraviolet radiation activated layer onto an alkali metal diffusion barrier layer deposited on a substrate by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d). Because the ultraviolet radiation activated layer is not intentionally heated during or after the sputtering, significant process simplification and cost reduction can be achieved.